Respiratory system

In humans and other animals, for example, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles.

Other animals, such as insects, have respiratory systems with very simple anatomical features, and in amphibians even the skin plays a vital role in gas exchange.

Plants also have respiratory systems but the directionality of gas exchange can be opposite to that in animals. The respiratory system in plants also includes anatomical features such as holes on the undersides of leaves known as stomata.

In mammals, the diaphragm divides the body cavity into the

  abdominal cavity: contains the viscera (e.g., stomach and intestines)

  thoracic cavity: contains the heart and lungs.

Respiratory tree: terminates in alveolus, alveoli. Respiratory bronchioles branch into alveolar ducts and into alveoli. Alveolus: microscopic air sacs, 300 million of these in human lungs. Total surface area large. Gas diffuses micrometer, very tiny distance.